1. Field of the Invention
This invention relates to optical apparatus for optimally focusing an image, and more particularly to apparatus for controlling the focus of an image in a camera.
2. Description of the Prior Art
Many concepts and techniques have been proposed in the prior art for detecting the focus of an optical system and for controlling the focus. Notwithstanding this, the determination of optimum focus for an optical system has continued to be difficult of achievement. In one form of apparatus that has been proposed in the prior art, the focus detecting system includes a spatial frequency filter which derives from the light output of a lens system a spatial frequency spectrum. Superimposed upon this spectrum is an amplitude modulation corresponding to scanning of the focus. There is thereby produced a fluctuating light flux pattern containing information concerning the focus. Focus information extracted from this pattern is employed to bring the image into a position of desired focus.
The spatial frequency filters of the known prior art apparatus have comprised rotating discs or drums having regularly spaced alternate transparent and opaque portions. Such discs or drums uniformally chop or interrupt, in the vicinity of the desired image plane, the components of the light beam that make up the image. When such a uniformly operating chopper or interrupter is used for the spatial frequency filter, it has been found that undesired changes in detected signal amplitude can occur, which changes give rise to spurious modulation and resulting error in the focus determination. This is believed to be due to signals that are produced by two, or several, subjects of nearly equal contrast but so positioned on the chopper that they produce out of phase signals for chopper positions where otherwise large amplitude signals would be expected.